When the optic nerve becomes diseased the doctor generally depends on what the patient can report about the acuity of his vision, his visual field, his color vision, etc. There are very few things the doctor can look at and measure directly. These objective signs are particularly valuable and are the subject of this study. Two groups of patients will be examined (a) optic neuritis, and b) ischemic optic neuropathy and the following measurements will be made: 1. Relative Afferent Pupillary Defect. This can be measured with a simple flashlight and a set of neutral density filters. 2. Pupil Cycle Time. Pupillary oscillations are induced at the slit lamp and the rate of the cycling is measured. 3. Latency of the Visual Evoked Response to a shifting checker-board stimulus. 4. Amplitude of the Visual Evoked Response to the same stimulus. We hope to learn how these signs vary in different kinds of optic nerve disease so that we can understand more about the normal functions of the optic nerve, and how these functions are disturbed in various kinds of diseases. This should lead to a clearer understanding of these common disease processes and hence to a more rational therapy. This way the patient with optic nerve disease will eventually benefit from this information.